1. Field of the Invention
This invention is directed to the deposition of silicon dioxide films by plasma enhanced chemical vapor deposition (PECVD) and more particularly, to an improved method for depositing high quality SiO.sub.2 films at high deposition rates.
2. Background of the Invention
Current trends in metal oxide semiconductor (MOS) technology are towards higher chip packing density, more complex devices with more process levels, and larger substrates. To accommodate these advances, MOS device dimensions must be scaled down and high temperature processing steps must be minimized. Future generations of MOS devices will require very high quality silicon dioxide (SiO.sub.2) gate dielectrics and low temperature processing. The deposition of dielectrics or insulators at low temperatures also has applications in a number of other semiconductor device technologies. For example, as secondary passivation layers, interlayer isolation, and lithographic masks in integrated circuits and also as primary gate dielectrics for thin film transistor (TFT) applications.
In addition, the modern trend is towards the use of single wafer deposition tools which necessitates very high deposition rates (at least 600-1000 angstroms per minute) for the tools to be commercially viable.
The conventional method for forming SiO.sub.2 gate dielectrics is to thermally grow the oxide films at temperatures from about 800.degree.-100.degree. C. The thermal oxide films have excellent electronic and mechanical qualities that have made this process the most widely used for conventional semiconductor transistor applications. However, in view of the above described new direction in manufacturing, there is a need for a method to deposit SiO.sub.2 films at low temperatures that have both electronic and mechanical qualities comparable to thermal oxides.
Plasma enhanced chemical vapor deposition (PECVD) is a technique which is used to deposit electronic materials at high rates and/or at low temperatures. Historically, however, oxide deposited at low temperatures has been far from electronic grade and although various properties of PECVD oxide have been reported in the literature, Adams, Solid State Technology, 26, 135 (1983), and Hollahan, J. Electro Chemical Society, 126, 933 (1979), there have been no reports of oxides with electrical characteristics approaching those deposited by conventional high temperature techniques. Thus, due to the fact that the electronic and physical characteristics of SiO.sub.2 films deposited by conventional PECVD are relatively poor, applications have been limited to those areas where film quality is relatively unimportant.
Recently, it has been shown that by modifying the PECVD technique, thin films of SiO.sub.2 of exceptionally high quality can be deposited at very low substrate temperatures (350.degree. C. or less). See, Batey et al., J. Appl. Phys., 60, 3136 (1986) and Batey et al., IEEE Electron Dev. Lett. EDL-8, 148 (1987). The Batey et al. technique combines very low flows of reactive gases with massive amounts of helium dilution and a low radio frequency (RF) power density. As a result, the deposition proceeds at a much reduced rate and film quality is much improved. Typical process parameters for the Batey et al. technique are: 2% SiH.sub.4 in He with a flow rate of about 20 sccm, N.sub.2 O with a flow rate of about 50 sccm, He with a flow rate of about 2,000 sccm or greater, pressure of about 1 Torr and an RF power density of about 0.02 W/cm.sup.2. The large amount of helium dilution insures uniformity and high quality. However, it was determined that the film properties depend strongly on the deposition rate, which had to be kept below a critical value of about 80 angstroms per minute or less. While good quality films were obtained, the deposition rate is too low for many applications, a problem which is magnified by the trends in the industry towards single wafer processing.
Another prior art method of depositing SiO.sub.2 films in a PECVD system is disclosed in U.S. Pat. No. 4,223,048 to Engle, Jr. Engle, Jr. is directed to a batch processing system which utilizes interleaved electrodes and vertical positioning of the wafers in order to improve the uniformity of the films. Engle, Jr. teaches a very high flow rate of N.sub.2 O (1,000 liters per minute), a flow rate of SiH.sub.4 of about 50 sccm and a flow rate of O.sub.2 of about 10-20 sccm. The plasma discharge is established by a low RF power (20 watts) and a deposition rate of approximately 500 angstroms per minute is achieved. While there is no mention of the electronic quality of the films, it is likely the electronic quality would be poor at such a high deposition rate. Moreover, the deposition rate still remains below that necessary for single wafer processing.